Standardize logging timestamp format

It would be great for gitlab.com, but honestly probably nice for everyone who uses our product if we could standardize on logging timestamps. %Y-%m-%dT%H:%M:%S.%N%Z is the dominant format.

• Gitaly 2018-05-23T13:41:10Z
• Pages 2018-05-23T11:14:50Z
• production_json 2018-05-23T15:24:23.915Z
• geo 2018-05-23T16:04:23.624Z
• sidekiq: 2018-05-23T14:40:00.808Z
• api_json: 2018-05-23T15:47:07.471Z
• gitlab-shell: 2018-05-23T15:45:56+00:00
• application.log May 23, 2018 09:56

This issue is for changing the timestamp format of gitlab-shell to the same as the rails production json log. See https://gitlab.com/gitlab-org/gitlab-ce/issues/47981 .